FR2787037A1 - METHOD AND DEVICE FOR REMOVING NITROGEN OXIDES FROM AN EXHAUST LINE OF AN INTERNAL COMBUSTION ENGINE - Google Patents

Abstract

- The present invention relates to a device for removing nitrogen oxides in an exhaust line of an internal combustion engine (1) operating in lean mixture, comprising a means (3) for trapping the nitrogen oxides, a means in order to regenerate said nitrogen oxides when the trapping means is saturated, a means (2) for treating hydrocarbons arranged upstream of the means (3) for trapping nitrogen oxides, a means (4) for injecting nitrogen oxides hydrocarbons placed upstream of the means (2) for treating the hydrocarbons, a means (7) for measuring the richness of the gases - According to the invention, the means (2) for treating the hydrocarbons is a partial oxidation catalyst (or spared) of the hydrocarbons which cooperates with said means (3) to trap nitrogen oxides and which makes it possible to obtain at its outlet gases having a low concentration of oxygen (O2) and high concentrations of carbon monoxide ( CO) as well as in hydrogen (H2) .- In addition, a means is provided for in recording and processing the data coming from the various sensors and / or stored so that an efficient regeneration of the NOx trap (3) is carried out without disturbing the operation of the engine.

Description

The present invention relates to the field of gas treatment.

  emitted from a mixture ignition engine

poor and diesel engines.

  Such engines indeed emit a certain number of pollutants which it is necessary to eliminate and this all the more effectively as standards are

  are prevalent especially in industrialized countries.

  Among the most numerous pollutants and the most troublesome for

  the environment, there may be mentioned nitrogen oxides.

  It is known to eliminate this type of pollutant by passing the exhaust gases through catalysts (called DeNOx) intended to convert nitrogen oxides. This requires post injection of reducers such as hydrocarbons for example. Since the known catalysts are active over a given temperature range, it may be necessary to place, in the pot

  catalytic, several catalysts having different formulations,

  say different activity ranges. This enlarges the field of action

catalytic elements.

  By way of illustration, the formulations used for low temperatures are of the Platinum / Alumina or Platinum / Zeolite type. The temperatures for which these catalysts are most active are

 C to 250 C.

  The so-called "high temperature" catalysts are generally active between 300 ° C. and 500 ° C. They are, for example, copper / zeolite type catalysts. However, in this context, a problem arises when the exhaust gases are not, at the level of the catalyst or catalysts, in a

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  temperature range for which the conversion of nitrogen oxides is sufficient. However, the overall effectiveness of such post-treatment systems remains limited. For example, a Platinum / Alumina catalyst with a post-injection of diesel commonly has an efficiency on oxides

less than 50% nitrogen.

  There are also catalysts on which the nitrogen oxides are adsorbed in different forms. For example, the NOx can be stored in the form of nitrates or be inserted into an oxide structure. These catalysts are commonly called "NOx traps". The NOx trap of the "nitrate" type is a catalyst which makes it possible to store nitrogen oxides on its surface, in an oxidizing medium. Generally it is composed of a precious metal deposited on or near a basic mass which is generally an oxide or a mixture of oxides of alkali, alkaline earth or rare earth. In a lean mixture (excess of oxygen), NO (NOx = NO + NO2) which is mainly found in the exhaust gases is oxidized by the precious metal to form NO2. This NO2 migrates to the surface of the catalyst to then adsorb on the oxide and form a nitrate. These nitrates are stable in an oxidizing medium over a very wide temperature range. To desorb these nitrates from the surface of the catalyst, it is necessary to rise to high temperature in an oxidizing medium or else

make a reducing mixture.

  Since the only thermal regeneration of the NOx trap does not make it possible to treat the nitrogen oxides reemitted at the exhaust, a second catalyst (of the continuous DeNOx type for example) is then necessary to reduce them.

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  Patent application EP-A1-0 540 280 describes a thermal regeneration of this type, with a NOx trap, fitted with a

  gas heating, followed by a nitrogen oxide reduction catalyst.

  The two catalysts are mounted in diversion from the main exhaust line. According to this document, a valve system makes it possible to reduce, during the destocking phases of the trap, the VVH (ratio between the gas flow rate and the volume of the catalyst which translates the time of contact of the gases with the catalyst). Thus, the NOx conversion rate on the nitrogen oxide reduction catalyst is improved. However, with this configuration, the part of the gas flow passing through the main duct does not

  pass through the NOx reduction catalyst.

  A regeneration by richness makes it possible to reduce the NOx desorbed by a three-way type catalysis by depositing on the catalyst a metal

  noble noble (rhodium for example).

  With a petrol engine operating in a lean mixture, the transition from poor to rich is compatible with its operating mode, on the other hand with a Diesel engine, obtaining a richness

  greater than 1 is more difficult to implement.

  A known implementation consists in injecting hydrocarbons into the exhaust line upstream of the catalyst, when the nitrates have to be desorbed from the NOx trap. US Patent 5,201,802 illustrates an embodiment of this type. However, although this method makes it possible to obtain riches momentarily greater than 1, the gaseous mixture obtained contains oxygen in a significant concentration,

  which is detrimental to regeneration.

  Another known method consists in re-injecting burnt gases at the intake, this at very high rates and in injecting hydrocarbons with

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  exhaust: document EP-A1-0 829 623 discloses a process of this type. The latter strategy has the disadvantage of disrupting the

  engine operation and make engine control more complex.

  The present invention includes a regeneration of a NOx trap, based essentially on the variation of the richness of the gases.

  exhaust; and which does not disturb the operation of the engine.

  The regeneration of a NOx trap here includes both destocking

  NOx and their reduction, the reduction being ensured by the NOx trap.

  More specifically, it is to allow the regeneration of the NOx trap by decreasing the oxygen concentration and by increasing the concentrations of carbon monoxide CO and hydrogen H2 in the exhaust gases upstream of the NOx trap. CO and H2 being by definition

  good reducing agents resulting from the partial oxidation of post-

  injected. Carbon monoxide acts both on destocking and on reduction. Thus the subject of the present invention is a device for eliminating nitrogen oxides in an exhaust line of an internal combustion engine operating in a lean mixture, comprising a means for trapping nitrogen oxides, a means for regenerating said oxides nitrogen when the trapping means is saturated, a hydrocarbon treatment means arranged upstream of the nitrogen oxide trapping means, a hydrocarbon injection means placed upstream of the

  treatment of hydrocarbons, a means of measuring the richness of gases.

  In accordance with the invention, the means for treating hydrocarbons is a catalyst for partial (or controlled) oxidation of

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  hydrocarbons which cooperate with said means for trapping nitrogen oxides which makes it possible to obtain at its outlet gases having a low oxygen concentration (02) and high concentrations of carbon monoxide (CO) as well as hydrogen (H2 ); the device according to the invention further comprises means for recording and processing the data from the various sensors and / or memorized so that regeneration is carried out

  efficient NOx trap without disturbing engine operation.

  According to one embodiment of the invention, the hydrocarbon injection means, the hydrocarbon treatment means and the NOx trapping means are arranged in this order and in series relative to the

  direction of gas flow in the exhaust line.

  According to a particular embodiment of the invention, the means for injecting hydrocarbons, the means for processing hydrocarbons and the means for trapping NOx are arranged in the main line.

exhaust itself.

  According to another possibility, the means for injecting hydrocarbons, the means for processing hydrocarbons and the means for trapping NOx are arranged in a bypass of the main exhaust line, the device according to the invention then comprising a way

  to modulate the gas flow between said branch and the main line.

  Without departing from the scope of the invention, a means for injecting hydrocarbons, a means for processing hydrocarbons and a means for trapping nitrogen oxides can thus be arranged both in the bypass and in the main line, with means for modulating the flow

  gases between said branch and the main line.

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  In addition, the device according to the invention can comprise at least one temperature sensor which can be arranged upstream of said means of

treatment of hydrocarbons.

  Furthermore, the device according to the invention may comprise a means for measuring the quantity of NOx trapped in the trapping means,

arranged downstream of the latter.

  In addition, a second temperature sensor can be placed in

  the exhaust line, downstream of the hydrocarbon processing means.

  In particular, the means of measuring the richness of the gases

  exhaust can be arranged downstream of the NOx trapping means.

  Without departing from the scope of the invention, the means for measuring the richness of the exhaust gases can be disposed between the means for processing

  hydrocarbons and the NOx trapping means.

  Furthermore, the exhaust line may include a means for preheating the gases placed upstream of the means for processing hydrocarbons. The present invention further relates to a process for removing nitrogen oxides in an exhaust line of an internal combustion engine operating in a lean mixture, characterized in that it consists in - trapping NOx in an appropriate means ; - injecting hydrocarbons into the exhaust line according to different engine operating parameters and the saturation state of a NOx trapping means; - partially oxidize the hydrocarbons in a specific way;

  - regenerate said NOx trapping means.

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  According to one aspect of the invention, the quantity of NOx stored is monitored

in the NOx trapping means.

  In accordance with another aspect of the invention, the temperature of the gases upstream and / or downstream of the hydrocarbon processing means is monitored. In addition, the richness of the gases upstream and / or downstream is monitored.

NOx trapping means.

  Advantageously, said additional hydrocarbons are injected when the NOx trapping means are saturated, the temperature (T1) of the exhaust gases is above a threshold value (TR) for which the hydrocarbon treatment means is active, so that the richness (X) of the exhaust gases is greater than or equal to a richness

  data (XR) which triggers the regeneration of the trapping means.

  More precisely, said hydrocarbons are injected for a period

  (dR) less than a stored, predetermined duration (dRMAX).

  According to an advantageous aspect of the invention, the flow rate of the

  exhaust gas between a main track and a bypass of said track.

  Thus, the exhaust gases are further heated before they are

  partial oxidation in said derivation.

  In particular, the regeneration is stopped as a function of the information given by a sensor such as a richness probe, placed in

downstream of the NOx trapping means.

  Other advantages, characteristics, details of the invention

  will appear better on reading the description which follows, made as

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  illustrative and in no way limitative with reference to the appended drawings in which: - Figure 1 is a diagram of an exhaust line according to an embodiment of the invention; - Figure 2 is a diagram of the organization of the data necessary for the method according to the invention; - Figure 3 is a flowchart relating to a method of regenerating a NOx trap according to the first embodiment of the invention; - Figure 4 is a diagram of an exhaust line according to a second embodiment of the invention; - Figure 5 is an illustration of the organization of data according to the second embodiment of the invention; and - Figure 6 is a flowchart relating to the regeneration process of a

  NOx trap according to the second embodiment of the invention.

  According to the embodiment of the invention illustrated by FIG. 1, the exhaust line of the engine 1 essentially comprises: a first catalyst 2 hereinafter called mild oxidation catalyst which makes it possible to carry out a partial oxidation of the hydrocarbons to monoxide of

carbon (CO) and hydrogen (H2).

  A second catalyst 3 is placed in the main exhaust line downstream of the first catalyst 2 relative to the direction of propagation of the gases in the exhaust line. This second catalyst 3 is a "NOx trap" whose regeneration mode (destocking and

  reduction) is done by wealth.

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  Furthermore, a means 4 for injecting hydrocarbons is provided for post-injection; the means 4 is arranged upstream of the two catalysts 2, 3. This post-injection can be provided by the engine injection system if possible: a common-rail type injection system, well known to specialists, can thus ensure such post-injection to the exhaust. A specific injection system, annexed to the injection in

  the engine itself can also be provided without departing from the scope of the invention.

  In addition, at least one temperature sensor may be required to

the implementation of the invention.

  In the embodiment of Figure 1, two temperature sensors 5, 6 are provided: one 5 located upstream of the controlled oxidation catalyst 2; the second temperature sensor 6 is placed

  between catalyst 2 and the NOx trap 3.

  The sensor 5 makes it possible to know whether the temperature of the gases is sufficient to oxidize the hydrocarbons injected in the regeneration phase of the

NOx trap 3.

  The temperature sensor 6 gives the temperature at which the NOx is stored in lean mixture and allows, during regeneration, to know the temperature variation due to post-injection.

of hydrocarbons.

  Downstream of the NOx trap 3, a NOx sensor 8 can be installed, intended to evaluate the quantity of NOx stored, at a given time, in the

NOx trap 3.

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  Finally, a richness probe 7 can be placed between the first and the second catalyst; this wealth sensor 7 can preferably be

  located downstream of the NOx trap 3.

  The operation of an exhaust line as it has just been described structurally is generally as follows: When the engine operates in a lean mixture, the catalyst or NOx trap 3 stores all or part of the NOx emitted by the engine 1 The fraction of NOx stored on the catalyst at time t depends both on the state of saturation in which the NOx 3 trap is located (ie what it has been able to store up to at time t) and conditions of temperature, flow rate, NOx concentration, exhaust gas richness. The amount of NOx stored on the catalyst can be estimated, either from data from the engine maps, and from a mathematical model or possibly using the NOx sensor 8 placed downstream of the NOx trap 3. When the quantity of NOx stored on the catalyst 3 reaches a threshold value S, the regeneration procedure is activated. The quantity of hydrocarbons which must be post injected in order to temporarily obtain a richness greater than 1 in the exhaust line is determined as a function of the parameters derived from the maps of the engine such as the richness and the flow rate of the exhaust gases. One can use for this purpose the proportional richness probe 7 placed for example downstream of the controlled oxidation catalyst 2 in order to loop on the post injection instruction. Furthermore, the temperature sensor 5 placed upstream of the first catalyst 2 indicates whether the temperature of the gases arriving on the catalyst

  controlled oxidation 2 is sufficient and therefore whether to activate post injection.

  - The controlled oxidation catalyst 2 then consumes the oxygen contained in the exhaust gases to form in particular carbon monoxide 2787037 carbon (CO) and hydrogen (H2), two elements which promote the

regeneration of the NOx trap 3.

  FIG. 2 is a summary of the parameters used according to the invention.

  We see that the engine 1 provides two types of data: the engine speed (N) and the position of the accelerator (o). From this data, a map C stored for example in an electronic control unit makes it possible to determine other parameters such as the richness R at the engine output, the NOx rate at the engine output and the exhaust flow rate. Other data, measured by the temperature sensor (s) 5, 6 or the NOx sensor 8, make it possible, during the NOx storage phase, to determine the quantity of NOx stored (Q NOX) at a given instant (t = n ) if

  they are associated with the first data.

  A mathematical model is used instead of or in addition to the NOx sensor 8, to determine the concentration of NOx in the gases. If a sensor 8 is used additionally to the mathematical model, it is then possible to permanently adjust the parameters of the model in order to follow the real behavior of the catalysts in the exhaust line; so we can for example take into account the

catalyst aging.

  In the regeneration phase, mapping C provides the same data as in the storage phase which, combined with temperature and richness measurements, make it possible to determine the flow

  of hydrocarbons to be injected into the exhaust.

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  FIG. 3 is a flow diagram of the different steps leading or

  no to the regeneration of the NOx trap 3.

  It appears that the quantity of NOx (Q NOx) stored in the NOx trap is constantly monitored; if this value reaches a certain threshold S stored in an electronic control unit, then the temperature T1 of the gases upstream of the first catalyst 2 is checked; if this temperature is higher than a threshold TR which corresponds to the minimum temperature for which the first catalyst 2 is active then we are interested in

  the richness X of the gases downstream of the first catalyst 2.

  The quantity of post-injected hydrocarbons is linked to the difference between the richness R of the gases at the engine outlet and the threshold richness AR necessary for

regeneration.

  At the end of regeneration, the injection of hydrocarbons is stopped and then

  starts the NOx storage process again.

  FIG. 4 relates to a second embodiment of the invention which differs from the first in that the injection means 4, the catalysts 2, 3 as well as the various temperature sensors 5, 6, richness 7 or NOx 8 are arranged not in the main exhaust line 10 but in a bypass 9. A valve or any other means intended to modulate the main flow of the exhaust gases from the engine 1 is

arranged at the source of the bypass 9.

  This configuration allows, compared to the first, to limit the

  gas flow through the catalysts during regeneration of the trap.

  The quantity of hydrocarbons to be post-injected is thus reduced to go to

wealth greater than 1.

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  During the NOx storage phase on the NOx trap, all of the exhaust gases pass through the conduit containing the catalysts. The

  post injection of hydrocarbon is not activated.

  The estimation of the quantity of NOx stored on the catalyst is identical to what has been described above. When the regeneration phase

  is activated, part of the gas is bypassed by means of the valve.

  This limits the flow through the catalysts which reduces

  the VVH (Hourly Volume Speed = Gas flow / Catalyst volume).

  The quantity of post-injected hydrocarbons is a function of the richness of the exhaust gases (provided by the engine maps) and the fraction of flow bypassed. The fraction of flow X in branch 9 is a function of

  total gas flow and valve position 11 at the head of branch 9.

  As before, it is possible to use a proportional richness probe placed downstream of the controlled oxidation catalyst or of the

  NOx 3 trap in order to complete the quantity of post-injected hydrocarbons.

  According to yet another embodiment of the invention, the exhaust line may further comprise a means 12 for heating the exhaust gases, arranged according to FIG. 4 in the branch 9, just in

  upstream of the controlled oxidation catalyst 2.

  The operating principle of this embodiment of

  the invention is generally the same as that which has just been described.

  The advantages of the third embodiment of the invention can be summarized as follows: As has been said previously, the post injection of hydrocarbons and therefore the regeneration of the NOx trap 3 is subject to the temperature level of the gases which pass through catalysts 2 and 3. Thus, if this level is insufficient, the regeneration of trap 3 cannot be activated to the detriment

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  the overall efficiency of the system. It may therefore be advantageous to add to the previous system, the device 12 making it possible to heat the gases in order to raise the temperature level during the regenerations when the latter is too low. The temperature sensors 5, 6 placed respectively upstream and downstream of the controlled oxidation catalyst 2 make it possible to manage the gas heating system. The advantage of bypass 9 is therefore twofold: reducing the quantity of hydrocarbons to be injected to increase the wealth to more than 1 and limiting the energy to be spent to heat up the fraction of

gas passing through the catalyst 2.

  In general, the advantage of the invention is to consume, during the regeneration of the NOx trap, the oxygen contained in the exhaust gases by the post-injected hydrocarbons on the controlled oxidation catalyst 2, and to form carbon monoxide CO and hydrogen H2 upstream of the NOx trap 3. In this way, during a post injection of hydrocarbons, a mixture is obtained upstream of the NOx trap which has a richness greater than 1, poor in oxygen and rich in CO and H2 which is favorable both to the desorption of the nitrates and to the reduction on the means (3) of the desorbed NOx. A notable improvement in the

  regeneration of the NOx 3 trap is thus obtained.

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Claims (17)

  1) Device for eliminating nitrogen oxides in an exhaust line of an internal combustion engine operating in lean mixture, comprising means (3) for trapping nitrogen oxides, means for regenerating said oxides of nitrogen when the trapping means is saturated, a means (2) for treating hydrocarbons disposed upstream of the means (3) for trapping nitrogen oxides, a means (4) for injecting hydrocarbons placed upstream means (2) for processing hydrocarbons, means (7) for measuring the richness of gases, characterized in that the means (2) for processing hydrocarbons is a catalyst for partial (or controlled) oxidation of hydrocarbons which cooperates with said means (3) for trapping nitrogen oxides and which makes it possible to obtain, at its outlet, gases having a low oxygen concentration (02) and high concentrations of carbon monoxide (CO) as well as hydrogen (H2), and in that it further comprises a means for entering record and process the data from the various sensors and / or memorized so that an efficient regeneration of the NOx trap (3) is carried out without disturbing the engine operation.   2) Device according to claim 1, characterized in that the means (4) for injecting hydrocarbons, the means (2) for processing hydrocarbons and the means (3) for trapping NOx are arranged in the line of escapement in this order and in series relative to the sense of gas circulation.   3) Device according to claim 2, characterized in that the means (4) for injecting hydrocarbons, the means (2) for processing 16 2787037   hydrocarbons and the means (3) for NOx trapping are arranged in the   main exhaust line itself.   4) Device according to any one of claims 1 to 3,   characterized in that the means (4) for injecting hydrocarbons, the means (2) for processing hydrocarbons and the means (3) for trapping NOx are arranged in a branch (9) of the main exhaust line , and in that it further comprises means (11) for modulating the gas flow   between said branch (9) and the main line (10).   ) Device according to any one of the preceding claims,   characterized in that it further comprises at least one sensor for temperature (5).   6) Device according to claim 5, characterized in that said temperature sensor (5) is arranged upstream of said means (2) of treatment of hydrocarbons.   7) Device according to any one of the preceding claims,   characterized in that it further comprises means (8) for measuring the quantity of NOx trapped in the trapping means (3) disposed downstream of the means (3) for trapping NOx,   8) Device according to any one of the preceding claims,   characterized in that it comprises a second temperature sensor (6)   placed downstream of said hydrocarbon processing means (2).   9) Device according to any one of claims 7 or 8,   characterized in that the means (7) for measuring the richness of the gases   exhaust is disposed downstream of the NOx trapping means (3). 17 2787037   ) Device according to any one of claims 7 to 8,   characterized in that the means (7) for measuring the richness of the exhaust gases is disposed between the means (2) for processing   hydrocarbons and the means (3) for trapping NOx.   11) Device according to claim 10, characterized in that the means (7) for measuring the richness of the gases is placed upstream of the   second temperature sensor (6).   12) Device according to any one of the preceding claims,   characterized in that it further comprises a means (12) for preheating   gases placed upstream of the means (2) for processing hydrocarbons.   13) Method for eliminating nitrogen oxides in an exhaust line of an internal combustion engine (1) operating in a lean mixture, characterized in that it consists in: - trapping the NOx in an appropriate means ( 3); - injecting hydrocarbons into the exhaust line as a function of different engine operating parameters and the state of saturation of a means (3) for trapping NOx; - partially oxidize the hydrocarbons in a specific means (2);   - regenerate said NOx trapping means (3).   14) Method according to claim 13, characterized in that the quantity of NOx stored in the means (3) for trapping NOx is monitored.   ) Method according to any one of claims 13 or 14,   characterized in that the temperature of the gases upstream and / or is monitored   downstream of the means (2) for processing hydrocarbons. 18 2787037   16) Method according to any one of claims 13 to 15,   characterized in that the richness of the gases is monitored upstream and / or in   downstream of the NOx trapping means (3).   17) Method according to any one of claims 13 to 16,   characterized in that said additional hydrocarbons are injected when the means (3) for trapping NOx is saturated, the temperature (T1) of the exhaust gases is greater than a threshold value (TR) for which the means (2 ) hydrocarbon processing is active, so that the richness (X,) of the exhaust gases is greater than or equal to a richness   data (AR) which triggers the regeneration of the trapping means (3).   18) Method according to any one of claims 13 to 17,   characterized in that said hydrocarbons are injected for a period   (dR) less than a stored, predetermined duration (dRMAX).   19) Method according to any one of claims 13 to 18,   characterized in that the exhaust gas flow is modulated between a   main track (10) and a bypass (9) of said track (10).   ) Method according to claim 19, characterized in that the exhaust gases are heated before their partial oxidation in said bypass (9).   21) Method according to any one of claims 13 to 20,   characterized in that the regeneration is stopped as a function of the information given by a sensor such as a richness probe, placed downstream of the NOx trapping means (3).